In the processing of semiconductor workpieces or "wafers" into integrated circuits, sputter etching is a technique that is often used to remove a layer of unwanted material from the wafer surface. The process of sputter etching is generally known and utilizes ionized particles of a charged gas plasma to bombard the surface of a wafer and dislodge or "sputter" away other substrate particles from the surface of the wafer. In sputter etching, a gas is introduced into a processing chamber which is preferably vacuum sealed and which may be typically fabricated from quartz. The wafer to be etched is supported on an electrically charged base or electrode within the reaction chamber whereon the wafer develops an electrical charge or bias. A work gas is introduced into the vacuum chamber opposite the surface of the charged wafer, and energy is inductively coupled to the gas through the processing chamber wall, such as by using an induction coil which surrounds the quartz processing chamber. The energy from the induced electric field ionizes the gas particles so that they acquire a net charge that is of the opposite polarity to the charge of the wafer support and the wafer. The ionized particles of the gas collectively form what is referred to as a gas plasma or plasma cloud. Since the ionized particles of the plasma and the wafer are of opposite polarities, the ionized particles in the plasma are attracted to the wafer surface, bombarding the surface of the wafer and dislodging material particles from the workpiece to, consequently, "etch" the wafer surface.
The sputter etching process commonly takes place at wafer voltages in the range of approximately 1,000 volts (1 kV). However, this relatively high voltage range is inappropriate for today's state-of-the-art microelectronic devices which are more susceptible to surface damage at these wafer charging voltages. As a result, lower wafer voltages, below 500 volts, are more desirable. Plasma etching that is accomplished using these lower wafer voltages is referred to as a "soft" plasma etch.
In a sputter etching process which utilizes an inductively coupled plasma as described above, the ionized plasma cloud that is formed in the proximity of the wafer often has an ion particle concentration or a "flux" concentration which is not perfectly uniform across the surface of the wafer. That is, the ionized plasma cloud may be more concentrated over one area of the wafer than over another area of the wafer. As a result of the nonuniformity of the plasma flux concentration, the etch rate on the surface of the wafer is generally nonuniform. For example, if the flux concentration is greater at the center of the cloud, the center of the wafer is etched at a higher rate than the peripheral edges. Nonuniformity in the flux concentration of the plasma may be due to various conditions, such as the relative concentration of the gas used to make the plasma cloud, the shape of the plasma, the wall effects of the processing chamber and any external electric or magnetic field effects around the chamber, as well as the effects of the electric field which ionizes the gas to create the plasma cloud within the chamber.
Therefore, it is an objective of the present invention to provide a more uniform etch rate across the surface of a wafer substrate, and thereby provide a more nearly uniform etch depth across the wafer from the center to the peripheral edge thereof. It is further an objective of the present invention to provide a selectively variable etch rate across the surface of the wafer to tailor the etch depth across the wafer.